| Popis: |
Earth and Moon were formed 4.6 billion years ago by the collision of the two protoplanets Gaia and Theia. Afterwards the Earth formed a crust where colliding comets provided the water of the oceans. This Hadean Eon was terminated 3.9 billion years ago during the Late Heavy Bombardment when an eccentric orbit of Jupiter caused a bombardment of the Earth by asteroids. Soon thereafter, 3.8 billion years ago, there is geochemical evidence of terrestrial life which performed photosynthesis. The terrestrial life witnessed and survived several cataclysms including the snowball Earth 760 to 580 million years ago, an ice age 440 million years ago which was possibly caused by a gamma-ray burst, and an asteroid impact 65 million years ago which generated the Chicxulub crater and contributed to the extinction of the dinosaurs. The 1908 Tunguska explosion was caused by a small stony asteroid. Amino acids, purines, pyrimidines and sugars, but no proteins, nucleotides or extraterrestrial unicellular organisms were detected in meteorites. This argues against the hypothesis of panspermia. The synthesis of amino acids, small peptides, purines and pyrimidine ribonucleotides under conditions of the primitive Earth (Stanley Miller experiments) and the polymerization of RNA nucleotides on clay minerals suggests that viroids and an RNA world could have existed during the Hadean Eon. |
